Controlled capacity torque converter clutch control system

ABSTRACT

A torque converter clutch control system has a plurality of operating modes among them an off mode wherein torque coupling is fluidic, a controlled capacity mode wherein the torque coupling is fluidic and mechanical during period of positive engine torque, an apply mode for smoothly transitioning from the OFF mode to the controlled capacity mode, and a release mode for smoothly transitioning from the controlled capacity mode to the off mode. Additionally, a lock mode is provided wherein torque coupling is mechanical via the torque converter clutch and a coast mode wherein torque coupling is mechanical and fluidic during periods of negative torque.

BACKGROUND OF THE INVENTION

This invention relates to a control of a torque converter clutch andmore particularly to a system for regulating the torque capacity of theclutch to control the torque converter slippage.

Introduced as an efficiency increasing device, the torque converterclutch is a fluid operated friction device engageable to couple theimpeller (input) and turbine (output) of a hydraulic torque converter.In the usual application, the clutch is either fully released to permitunrestrained slippage between the impeller and the turbine or fullyengaged to prevent such slippage entirely. An unfortunate aspect of fullconverter clutch engagement is that the engine torque perturbations ortorsionals normally absorbed by the torque converter are passed directlythrough the clutch to the vehicle drivetrain and may produce annoyingpulsations therein if not properly damped. This factor operates torestrict the usage of the torque converter clutch to specified vehicleoperating conditions for which the annoying effects are minimized. As aresult, the potential efficiency gains afforded by engagement of thetorque converter clutch have only been realized over a portion of therange of vehicle operation.

To overcome the disadvantages of-torque converter clutch engagement, ithas been proposed to operate the clutch in a slipping mode wherein apredetermined amount of slippage between the torque converter impellerand turbine is permitted for regulating the torque capacity of theclutch. In any such system, the objective is to isolate engine torqueperturbations in the torque converter while passing steady state enginetorque at a slip rate that provides improved torque converterefficiency. One such system that controls the clutch slippage to achievethe above objectives is disclosed in U.S. Pat. No. 4,582,185 to Grimeset al., issued Apr. 15, 1986 and assigned to the assignee of the presentinvention.

Generally speaking, the system identified above operates to generateclutch engagement force without regard to the magnitude of the sliperror. Whenever the measured slip is greater than the desired slip, thecontroller acts to increase the clutch engagement force to increase thetorque capacity of the clutch. Whenever the measured slip is less thanthe desired slip, the controller acts to decrease the clutch engagementforce to decrease the torque capacity of the clutch.

While the control of the torque converter clutch in a slip mode may bedesirable over a significant range of various driving conditions,further refinements to the torque converter clutch control can be madeto improve overall efficiency. For example, full torque converter clutchengagement whereby slip is substantially zero may be acceptable duringperiods of vehicle operation where torque disturbances are not expectedto be objectionable. Typically, highway cruising with nominal vehicleloading and sufficient engine speed may benefit from such fullengagement of the torque converter clutch. However, as loading andengine speed vary, torque disturbances may become objectionable andrelease of full engagement desirable. Both the application and releaseof a torque converter clutch into or out of a full engagement maythemselves become objectionable particularly where engine load or speedconditions vary.

SUMMARY OF THE INVENTION

The present invention is directed toward a control for a torqueconverter clutch (TCC). In particular, the control is operative toeffectuate operation within and transitions between a plurality ofoperating modes. In general, the control comprises: an OFF mode whereinthe TCC is fully disengaged and all torque transfer between the inputand output members is fluidic; and APPLY mode wherein the TCC gainscapacity and torque transfer between the input and output members ismechanical and fluidic; a controlled capacity mode (CC mode) wherein apredetermined amount of slip is maintained at the TCC and positivetorque coupling is mechanical and fluidic; and, a RELEASE mode whereinthe TCC loses capacity and torque transfer between the input and outputmembers is mechanical and fluidic. Additionally, the control comprises aLOCK mode wherein positive torque coupling is mechanical and a COASTmode wherein positive and negative torque coupling is mechanical andfluidic.

In accordance with a first aspect of the present invention, a controlmeans for establishing a pressure control signal has associatedtherewith a plurality of inputs used to determine the desired mode ofoperation. An off mode of operation provides for a minimal pressuredifferential across the clutch actuating mechanism to thereby providesubstantially zero mechanical torque capacity of the TCC. The APPLY modeis operable to smoothly transition from the OFF mode to the CC mode byincreasing the torque capacity of the TCC during periods of positivetorque. The CC mode is operative after the APPLY mode has diminished theslip across the TCC and effectively controls the slip to a predeterminedreference slip by varying the pressure control signal in response toboth positive engine torque and slip deviation from a reference slip.The RELEASE mode is operable to smoothly transition from the CC mode tothe OFF mode by decreasing the torque capacity of the TCC.

According to another aspect of the present invention, a LOCK mode ofoperation is accessible from the CC mode to provide a pressure controlsignal which varies in response to engine torque to maintain torquecapacity at least as great as the torque being transmitted between theinput and output of the torque converter. Further, a controlled releasemay be instituted from the LOCK mode to smoothly transition into the OFFmode.

Another aspect of the invention provides for TCC capacity control duringperiods of vehicle coast wherein torque being transferred may benegative. Preferably, torque capacity is controlled in response to adrivetrain member speed quantity.

Yet another aspect of the present invention provides for an APPLY modewherein the pressure control signal is established as the summation of aquantity established in accordance with the engine torque and varying inrelation thereto thereby contributing either positively or negatively tothe pressure control signal and a signal which varies in a singledirection only to contribute positively to the pressure control signal.The result of APPLY mode control in this fashion ensures that steadystate torque variations during the apply will neither increase the applytime to unacceptable levels nor cause too swift an application orpotential unintended lock-up.

Still another aspect of the present invention adapts the pressurecontrol signal to transient throttle conditions by summing therewith aquantity corresponding thereto which contributes negatively to thetorque capacity whereby during aggressive positive throttle andaggressive negative throttle an additional margin of slip is provided toallow for increased fluid coupling and decreased likelihood ofunintended lock-up respectively.

These and other aspects of the present invention are more thoroughlyunderstood by the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic and cross-sectional diagram depicting the torqueconverter clutch, certain transmission fluid handling elements, and amicrocomputer-based control unit for carrying out the control functionsof this invention.

FIGS. 2-15 are flow diagrams representative of program instructionsexecuted by the control unit shown in FIG. 1 in carrying out the controlfunctions of the present invention.

FIGS. 16A and 16B are time based graphs illustrating the operation ofthe control system with respect to clutch application and controlledcapacity operation.

FIGS. 17A and 17B are time based graphs illustrating the operation ofthe control system with respect to a transition from controlled capacityoperation to total clutch disengagement,

FIGS. 18A, 18B and 18C are time based graphs depicting the operation ofthe control system with respect to torque converter clutch transitionsbetween a controlled capacity mode and a LOCK mode,

FIGS. 19A and 19B depict the operation of the control system withrespect to the torque converter clutch transitions into and out of LOCKmode and controlled capacity mode of operation,

FIGS. 20A, 20B and 2DC depict the operation of the control system withrespect to transitions into and out of a COAST mode and controloperation within the COAST mode,

FIGS. 21A, 21B and 21C depict the operation of the control system withrespect to certain transitions of the torque converter clutch from anoff mode of operation into a controlled capacity mode of operation.

FIGS. 22A, 22B and 22C depict the operation of the control system withrespect to certain high rates of change in throttle position.

FIG. 23 illustrates an overall control system including modes ofoperation and transition paths according to the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now to FIG. 1, reference numeral 10 generally designates aportion of an automatic transmission including a conventional fluidictorque converter 12 disposed within the transmission housing 14. theimpeller or input member 16 of torque converter 12 is connected to berotatably driven by the output shaft 18 of engine 20 through the inputshell 22, as indicated by the dashed line 24. The turbine or outputmember 26 of the torque converter 12 is rotatably driven by the impeller16 by means of fluid transfer therebetween, and is connected torotatably drive the torque converter output shaft 28 through a splinedhub member 30. The stator 32 redirects the fluid coupling impeller 16 tothe turbine 26 and is connected through a one-way device 34 and asplined sleeve shaft 36 to the transmission housing 14. The converteroutput shaft 28 is mechanically coupled to a suitable gear set forproviding a plurality of distinct speed ratios between the converteroutput shaft 28 and the transmission output shaft.

Also disposed within the transmission housing 14 is a torque converterclutch (TCC) assembly, generally designated by the reference numeral 50.Clutch 50 includes a clutch plate 52 having a friction surface 54 and adrive disc 56 coupled for rotation to clutch plate 52 by rivet connector58. The drive disc 56 and plate 52 are axially slidable on hub member30, and the drive disc 56 is splined onto hub member 30 so that theengagement of friction surface 54 of clutch plate 52 with the inputshell 22 provides a mechanical drive between the input shell 22 and theconverter output shaft 28.

Clutch plate 52 divides the space between turbine 26 and input shell 22into two fluid chambers; an apply chamber 60 and a release chamber 62.When the fluid pressure in the apply chamber 60 exceeds that in therelease chamber 62, there is a resultant force which tends to move thefriction surface 54 into engagement with input shell 22. Such forcetends to reduce the slippage between the impeller 16 and turbine 26 andwhen sufficiently great, fully engages the clutch 50 to prevent suchslippage entirely. When the fluid pressure in release chamber 62 exceedsthat in apply chamber 60, there is a resultant force which tends to movethe friction surface 54 out of engagement with input shell 22. Suchforce tends to increase the slippage between impeller 16 and turbine 26,and when sufficiently great fully releases the clutch 50 to permitunrestrained slippage therebetween. The vehicle drivetrain as referencedherein shall include all mechanical elements from the output member 26and/or clutch plate 52 through the final drive components (not shown).

The input shell 22 has splined thereto a pump drive shaft, not shown,which extends coaxial with and through converter output shaft 28 andwhich is mechanically connected to drive a positive displacementhydraulic pump (not shown). The pump supplies hydraulic fluid from afluid reservoir to the torque converter 12, the transmission controlvalves, the various clutches and brakes of the transmission gear set,and the transmission lubrication and cooling systems. The pump ispreferably of a variable displacement design, such as that shown in theU.S. Pat. No. 4,342,545 to Schuster issued Aug. 3, 1982, and assigned tothe assignee of the present invention; however, a fixed displacementpump will perform satisfactorily. A pressure regulator valve regulatesthe fluid pressure from the pump (hereinafter referred to as linepressure). The foregoing description of TCC mechanics and operation aregenerally well known in the art.

The control system of this invention operates as described below tocontrol the fluid pressure in the apply and release chambers 60 and 62to control the amount of slippage between the impeller 16 and theturbine 26. As such, the remainder of the elements depicted in FIG. 1are devoted at least in part to such purpose.

The regulator valve 70 comprises an integral solenoid valve 72 and spoolvalve 74, the spool 76 being linearly displaceable within the valve bore78 through energization of the solenoid coil. A spring 82 biases thespool 76 leftward to the position shown in FIG. 1. When the solenoidcoil is energized, chamber 80 is pressurized to overcome the springforce thereby displacing the spool to the rightward position.

The inlet port 86 is supplied with the transmission main or linepressure, and the port 88 is a controlled pressure port. When the spool76 is in the spring biases position as illustrated in FIG. 1, the spoolland 94 blocks the controlled pressure port 88 and the inlet port 86 isdecoupled therefrom. When the spool valve 76 is in the solenoid valvebiased position, the spool land 94 no longer blocks the controlledpressure port 88, and the line pressure port 86 is coupled thereto. Inoperation, the solenoid coil is pulse-width modulated (PWMed) at avariable duty cycle to shift the spool 76 between the two positions toeffect a ratiometric pressure control as explained below.

Control valve 100 connects the regulator valve 70 to the apply andrelease chambers 60 and 62 of the torque converter clutch mechanism 50.The controlled pressure port 88 is connected to the control valve port102 via line 103. The control valve ports 110 and 112 are connected tothe apply and release chambers respectively. Line 106 is connected to abias chamber 122 of directional valve 100 so that the pressure in line106 acts on the land 124 of spool 126. A spring 128 disposed within thespool sleeve 130 exerts a rightward force in opposition to the leftwardforce of the pressure in bias chamber 122. The area of the valvesurrounding spring 128 is exhausted via exhaust port 132.

When the solenoid coil of solenoid valve 72 is completely deenergized,lines 106 and chamber 80 are exhausted through port 98, line 96 isdisconnected from therefrom and spool 126 assumes the spring biasedposition illustrated in FIG. 1. In this configuration, a regulatedconverter feed pressure in line 134 is supplied to the clutch releasechamber 62 via control valve port 112 and line 116, and the applychamber pressure is diverted to the cooler (not shown) via port 120.This moves the clutch plate 52 away from the input shell 22, disengagingclutch 50 and supplying operating pressure to the torque converterimpeller 16. In this state, normal torque converter operation isachieved.

When it is desired to apply the torque converter clutch 50, the solenoidcoil of solenoid valve 72 is intermittently energized by PWM. Thisproduces a PWM pressure in line 106 in accordance with the pressure inline 96 and the PWM duty cycle. When the PWM duty cycle is relativelylow, the resulting pressure in bias chamber 122 of control valve 100 isinsufficient to overcome the bias force of spring 128. Consequently, thecontrol valve is maintained in the release position (top position) ofFIG. 1, and normal torque converter operation continues.

If the PWM duty cycle is increased, the pressure in the bias chamber 122also increases. When the bias pressure force on land 124 exceeds thespring force, the spool 126 moves leftward to the position depicted bythe lower position of the spool in FIG. 1. The pressure and PWM dutycycle which produce the position change of the control valve 100 arereferred to herein as the switch point pressure and switch point dutycycle.

In the control state configuration, the regulated converter feedpressure in line 134 is directed via port 111 and 120 to the cooler (notshown) and the controlled pressure is supplied to impeller 16 and clutchapply chamber 60 via control valve ports 102 and 104, lines 103 and 110,the regulator valve port 88 and valve bore 78. At the same time, thepressure in release chamber 62 is connected to the exhaust port 92 viacontrol valve ports 112 and 114. At PWM duty cycles just above theswitch point duty cycle, the apply chamber pressure is relatively low,resulting in relatively low net clutch engagement pressure. Atrelatively high PWM duty cycles, the apply chamber pressure isrelatively high, resulting in relatively high net clutch engagementpressure.

The energization of the solenoid coil is controlled by an electronictransmission control unit (TCU) 180 via line 182. The control is made inresponse to a number of input signals including a brake signal (BR) online 186, an engine throttle signal (% TH) on line 187, a transmissiongear range signals (RANGE) on lines 188, an engine speed signal (Ne) online 189, a turbine speed signal (Nt) on line 190 and a transmissionoutput speed signal N(o) on line 191. The brake signal may be obtainedwith a switch mechanism (not shown) responsive to movement of thevehicle brake pedal such that depression of the bake pedal causes achange in the output state of the brake signal. The engine throttlesignal may be obtained with a suitable transducer, such as a rotarypotentiometer (not shown) responsive to the position of the acceleratorpedal or engine throttle for producing an electrical output signal inaccordance therewith. The transmission gear range signals may beobtained with suitable pressure transducers (not shown) located withrespect to the fluid conducting passages of the transmission 10 in amanner to determine the gear range selected. The turbine speed (Nt),engine speed (Ne) and output speed (No) signals are obtained fromconventional speed transducers such as the variable reluctance typewhich cooperate with magnetic gear teeth formed on the surface of arotating shaft.

The (TCU) 180 essentially comprises a microcomputer (μC), aninput/output (I/O) device, which communicates with the microcomputer viaan address and control bus and a bi-directional data bus, and a highfrequency clock for supplying the microcomputer with a high frequencypulse train for controlling the operational timing of the same. Themicrocomputer is preferably of the type including internalRandom-Access-Memory (RAM), Read-Only-Memory (ROM) and timing circuitry.The brake, throttle, gear, engine speed, turbine speed and output speedsignals on lines 186, 187, 188, 189, 190 and 191 are applied as inputsto the input/output device, and the input/output device includescircuitry for converting analog input signals to a digital format andfor developing suitable control signals on line 182 for controlling theenergization of the solenoid coil in response to duty cycle commandsdeveloped by the microcomputer. A flow diagram representative ofsuitable program instructions for performing the control functions ofthis invention is given in FIGS. 2-15.

The graphs shown in FIGS. 16-22 depict various features of the controlof the present invention. In all figures wherein slip (ΔN) isillustrated, positive slip is shown by a solid trace above the zero axesand is indicative of the torque converter input member having arotational speed in excess of the torque converter output member. Thisis the situation normally expected when steady state engine torque(hereinafter engine torque) is being provided to the vehicle drivetrainwhen the TCC is either disengaged or partially engaged. Torque in thesesituations is said to be positive. When slip is indicated as beingnegative as shown by a solid trace below the zero axis line in anyfigure, the torque converter output member has a rotational speed inexcess of the torque converter input member. This is a condition foundthrough periods of vehicle operation wherein the TCC is disengaged orpartially engaged and the vehicle drivetrain is to some degree drivingthe vehicle engine. Torque in these situations is said to be negative.

The various pressure charts shown throughout FIGS. 16-22 depict the netfluid pressure in a control chamber of the TCC which is used to maintaina controlled degree of TCC engagement. The net fluid pressurerepresented by the solid lines labeled PTCC in the Figures, depending onthe mode of operation, may be comprised of summations of variouspressure quantities as will be developed at further points herein. Thesepressure graphs interchangeably may represent actual pressure within thecontrol chamber of the TCC or a pressure control signal, for example avariable duty cycle supplied to a clutch controlled mechanism asillustrated in FIG. 1 previously described.

Beginning with reference to FIG. 23, a block diagram representation ofthe various modes of operation of an overall control system for the TCCis shown. The boxes 2301-2311 depict the various modes of operationavailable to the control system. The lines interconnecting the variousboxes represent the transitions between modes of operation available tothe control system. In the present control, the OFF mode represented byblock 2301 is a priority mode which is selected when any one of avariety of OFF mode criteria would so indicate the desirability ofcomplete TCC disengagement. For example, until a vehicle is operating ina steady state condition with respect to closed loop engine control atan appropriately high engine temperature, the OFF mode would be selectedand engagement of the TCC would not be allowed even if other criteriaotherwise associated with TCC application would so indicate itsengagement. The remaining various modes of operation each havepriorities of a lesser degree than the OFF mode, the various degrees ofpriority being represented in conjunction with the discussion of flowcharts of FIGS. 2-15.

Assuming the OFF mode is a starting point for discussion of the overallsystem and transitions between modes, at a time when it is appropriateto exit the OFF mode wherein the TCC is completely disengaged and entera mode of operation wherein the TCC is partially or fully engaged, thecontrol of the present invention operates to allow TCC engagementthrough an APPLY mode of operation as represented by block 2303. Duringthe APPLY mode of operation, the net fluid pressure in the controlchamber in the TCC is increased at a predetermined rate until positiveslip across the TCC reaches a predetermined slip threshold. After thatthreshold is reached, the CONTROLLED CAPACITY mode (hereinafter "CCmode") of operation as represented by block 2307 is entered. If at anytime during the application of TCC via APPLY mode 2303 the OFF mode ischosen by any of the various criteria indicating the desirability of atotal and immediate disengagement of the TCC, then the OFF mode 2301 isonce again selected. Likewise, if during the APPLY mode 2303, theRELEASE mode is chosen by any of the various criteria indicating thedesirability of a controlled release of the TCC, the RELEASE mode 2305is selected.

Assuming operation within the CC mode 2307, the positive slip across theTCC is controlled by varying the net fluid pressure in the controlchamber of the TCC in response to deviations in the measured slip from apredetermined reference slip. Preferably, the reference slip isdetermined as a function of a drivetrain member speed quantity. One suchreadily available quantity is at the torque converter output, orturbine, which advantageously has associated therewith the turbine speedsignal (Nt). Alternatively, the transmission output speed signal (No)could be utilized but is not the preferred drivetrain member speedquantity. Where measured slip exceeds the reference slip, the control ofthe present invention serves to vary the fluid pressure in the controlchamber to increase torque capacity of the TCC thereby reducing themeasured slip toward the reference slip. All else being equal, thistranslates into an increase in the net fluid pressure within the controlchamber. When the measured slip is less than the reference slip, thecontrol of the present invention serves to vary the fluid pressure inthe control chamber to decrease torque capacity of the TCC therebyincreasing the measured slip toward the reference slip. All else beingequal, this translates into a decrease in the net fluid pressure in thecontrol chamber. This control methodology is described in detail in U.S.Pat. No. 4,582,185 also assigned to the assignee of the presentapplication. This is an appropriate control scheme if the engine torqueis substantially constant throughout the CC mode of operation. Such anengine torque situation would result in equivalent pressure changes pereach control loop which, when accumulated over multiple control loops,results in a substantially linear ramp up and ramp down of net fluidpressure in response to clutch slip greater than and less than thereference slip respectively. However, experience has shown significantimpact upon the ability of such a system to adequately control slipduring periods wherein the engine torque deviates. For example, whereengine torque increases significantly during a CC mode of operation,control of the torque converter slip by simple equivalent stepadjustments may prove to be inadequate in controlling slip to thereference slip. Likewise, significant reductions in engine torque duringperiods of CC modes of operation may cause undesirable lockup of the TCCfor similar reasons. The present invention therefore advantageouslyprovides for a net fluid pressure in the control chamber of the TCCwhich is comprised of a first portion which has a predeterminedcorrelation to engine torque and is responsive thereto, and a secondportion which is a function of the slip deviation from the referenceslip. Any of a variety of measurements of engine torque can be utilizedincluding engine torque estimations based upon throttle position andcalibration table look-ups, or real time engine torque calculationsbased on well known engine volumetric efficiency techniques.

Under circumstances appropriate for full application of the TCC, a LOCKmode of operation 2309 is entered. It is apparent from FIG. 23 that LOCKmode operation 2309 is accessible only from a CC mode of operation.While in the CC mode of operation, if measured slip remains less than apredetermined threshold for an appropriate amount of time, LOCK mode iscause to be entered. In accordance with the present embodiment, theinitial net fluid pressure in the control chamber at the onset of theLOCK mode is equivalent to the terminal net fluid pressure at the end ofthe CC mode with the addition thereto of a fixed fluid pressure stepthereby resulting in a net fluid pressure in the control chambersufficient to increase the capacity of the TCC such that the slip isreduced to, and maintained at, zero. The fluid pressure step is chosensuch that the net fluid pressure resulting from its addition to theterminal net fluid pressure in the CC mode will be marginally sufficientto cause full clutch engagement and zero slip speed. With such acalibrated amount of additional fluid pressure being marginallysufficient to reduce slip to zero, substantially large torqueperturbations experienced during LOCK mode operation, which with alarger fluid pressure step would cause undesirable transmission of thetorque perturbations through to the drivetrain, may advantageously bebypassed by virtue of the marginal fluid pressure step.

Preferably, while in the LOCK mode, the net fluid pressure is comprisedof the summation of a pressure which has a predetermined correlation tosteady state engine torque and a pressure which is a fixed valueequivalent to the summation of a frozen terminal value of theaccumulated fluid pressure adjustments made in the CC mode and the fluidpressure step. The net fluid pressure in the lock thereby varies inaccordance with steady state engine torque variations. Upon conditionsindicative of the desirability of returning to the CC mode from the LOCKmode of, the lock step fluid pressure is removed from the net fluidpressure at the termination of the LOCK mode thereby resulting in aninitial net fluid pressure in the CC mode comprised of a portion havinga predetermined correlation to engine torque and a portion equivalent tothe frozen terminal value of the accumulation of fluid pressureadjustments made in the previous CC mode. Accumulation of fluid pressureadjustments will continue in the CC mode beginning with the frozenterminal value and accumulating adjustments thereto.

Yet another mode of operation, a COAST mode 2311, is available foraccess from the CC mode or the LOCK mode. Conventional TCC control wouldrelease fully the TCC in the event of moderate throttle releases. Thecontrol of the present invention provides for a COAST mode whereintorque capacity is controlled between the input and output members ofthe TCC during periods of partial or full throttle releases. Throughthese periods of vehicle operation, a couple of clear advantages areprovided by such a control method. Firstly, through periods of vehiclecoast wherein the operator resumes the previous throttle setting, sinceslip is being controlled to a predetermined slip there is no need tocycle through an APPLY mode and transition to CC mode is more direct andefficient. Secondly, where the vehicle driveline is coupled to thevehicle engine thereby providing negative torque thereto, engine fuelingrequirements for sustaining idle are significantly reduced. The COASTmode therefore contributes to fuel economy by reducing idle fuelrequirements and providing for increased operation time in the CC mode.If the TCC were fully applied through periods of vehicle coast,undesirable driveline disturbances would be introduced when the operatorresumes a positive torque producing throttle setting. Therefore, withoutthe control of the present invention whereby TCC capacity is controlledduring periods of vehicle coast, the fuel economy afforded thereby wouldbe available only at the expense of undesirable driveline disturbances.

From FIG. 23, it is clear that the OFF mode, as previously mentioned ashaving priority over all other modes, is accessible from all of thevarious modes of operation. Additionally, the RELEASE mode is accessiblefrom all modes wherein the TCC has some degree of torque carryingcapacity.

Referring to FIGS. 16A and 16B, the measured slip ΔN and net fluidpressure PTCC in the APPLY mode are graphically depicted therein againstcommon time scales. The graph of FIG. 16A depicts measured clutch slipΔN and FIG. 16B depicts net fluid pressure PTCC in the control chamberof the TCC. Beginning in the OFF mode, it can be seen that slip hasattained a relatively steady value which is greater than the desiredslip labelled ΔNref. It is also noted here that the measured slip in theOFF mode is greater than the OFF mode slip threshold ΔNoff. Assumingthat all other OFF mode exit criteria have been satisfied, applicationof the TCC can begin provided that APPLY mode entry criteria have beensatisfied, including attainment of an appropriate vehicle speed for thecurrent gear. This determination preferably is performed via a tablelook-up of apply line calibrations in a manner well known to thoseskilled in the art. Such apply line calibration for TCC applicationdiffer from conventional apply line calibrations in that in addition tothe top gears, such as the third and fourth gears in a conventional fourspeed automatic transmission, lower gear(s) would be also represented byappropriate apply line calibration table(s) thus extending fuel economybenefits of TCC application to lower gears and vehicle speeds.

Beginning at time tl as illustrated in FIGS. 16A and 16B, the APPLY modeis seen to continue until time t2 whereupon the CC mode is entered. Attime t1, the net fluid pressure PTCC in the apply chamber of the TCC isseen to increase from zero pressure to an initial pressure equivalent toa pressure having a predetermined correlation to engine torque. Thebroken line (Ptq) represents the portion of PTCC established as afunction of engine torque. At time t1 it is seen that the pressure valuePtq is equivalent to PTCC, and indeed comprises the entire fluidpressure in the control chamber. The portion of the net fluid pressurerepresented as a function of engine torque Ptq represents an APPLY modebaseline fluid pressure to which is added an accumulation ofmagnitudinally and algebraically equivalent fluid pressure adjustmentsthroughout the APPLY mode. The APPLY mode is normally terminated whenthe measured slip ΔN is less than or equal to the APPLY mode slipthreshold established as a summation of the reference slip ΔNref and theAPPLY mode slip offset ΔNapp.

During the APPLY mode, the release of throttle position beyond a certainamount which is likely to reduce positive engine torque significantly orcause torque to be negative would indicate the desirability ofterminating the APPLY mode. Terminating continued TCC application duringthese minimal throttle positions will prevent unintended lock-up andassociated driveline disturbances. For this purpose, a reduction inthrottle position beyond a coast throttle threshold (% THcstL) willinvoke the OFF mode immediately from the APPLY mode.

At time t2 the initial net fluid pressure value in the CC mode isequivalent to the terminal value of the net fluid pressure in the APPLYmode. The net fluid pressure PTCC continue to be comprised of a portioncorresponding to a measure of engine torque Ptq and an accumulation offluid pressure adjustments; however, the fluid pressure adjustments areof a lesser magnitude in the CC mode compared with the fluid pressureadjustments in the APPLY mode. Therefore, it can be seen that whereengine torque is substantially constant, the net fluid pressure changein the APPLY mode occurs at a greater rate than the fluid pressurechange in the CC mode. Additionally, the fluid pressure adjustments arebidirectional providing additions to and subtractions from net fluidpressure.

In examining the far right portions of FIG. 16B beginning at time t3, itcan be seen that the portion of net fluid pressure in the apply chamberwhich is a function of engine torque undergoes a change in response to acorresponding change in engine torque. The correlation between thechange in engine torque and a concomitant change in net fluid pressuremaintains measured slip substantially at the reference slip while in theCC mode. Increases in engine torque therefore are accounted for in thenet fluid pressure as established by the summation of a portion thereofcorrelating to the engine torque and a portion comprising theaccumulation of fluid pressure adjustments in response to the sense ofthe measured slip versus the reference slip.

FIGS. 21A-21C depict a situation wherein a more aggressive applicationof the TCC may be desirable during CC mode. CC mode is seen terminatedat time t1 by a minimum throttle condition where throttle position fallsbelow a first minimum throttle threshold % THminL. Negative slip ANthrough the OFF mode is indicative of vehicle coast wherein wheel torqueis transmitted through the vehicle drivetrain to the output member ofthe torque converter. Through the OFF mode, relatively little resistanceto engine acceleration is presented by the strictly fluid couplingbetween the input and output members of the torque converter. Uponincreasing the throttle position, slip ΔN can be seen to increaserelatively rapidly due in part to the insubstantial resistance to engineacceleration. A second minimum throttle threshold % THminH (which isshown occurring prior to time t2) is ultimately exceeded therebyenabling OFF mode termination if APPLY mode criteria are satisfied.

Proceeding with the assumption that vehicle speed is in excess of theappropriate apply line, time t2 marks the initiation of the APPLY modedue to slip ΔN exceeding the OFF mode slip threshold ΔNoff. At time t3which marks a single control loop from t2, CC mode is invoked becausemeasured slip ΔN is less than or equal to the APPLY mode slip threshold(ΔNref+ΔNapp). This of course reduces the rate at which fluid pressureadjustments are made since, as previously described, a less aggressiveadjustment is associated with the CC mode than with the APPLY mode.However, acceleration of the engine continues and, without a moreaggressive adjustment to fluid pressure, the slip ΔN may becomeexcessive and require a lengthy reduction time toward the referenceslip. Therefore, it is desirable to control such potentially excessiveslip and the present embodiment does so by substituting the pressureadjustments normally associated with the APPLY mode in the CC mode. Thesubstitution is invoked by slip ΔN being in excess of the APPLY modethreshold (ΔNref+ΔNapp) as shown at time t4 within a predetermined timeafter CC mode is entered. The accumulation of APPLY mode fluid pressureadjustments continues through the CC mode until slip falls below thethreshold as illustrated at time t5, whereafter the CC mode fluidpressure adjustments are accumulated.

Referring now to FIGS. 17A and 17B, transition out of the CC mode andinto the OFF mode via a controlled release is illustrated. The measuredslip ΔN and net fluid pressure PTCC are graphically depicted thereinagainst common time scales. Assuming steady state operation in the CCmode, a controlled release is initiated when RELEASE mode entry criteriahave been satisfied. Such criteria preferably includes reduction to anappropriate vehicle speed for the current gear as determined in aconventional fashion via a table look-up of release line calibrations.Of course, lower gear release line calibration tables are necessarilyutilized where the control of the present invention operates inconjunction with lower gears. Positive change in throttle position isyet another criteria which is used to initiate a controlled release bythe RELEASE mode. The time rate of change in throttle position mayprovide indicia of the operator's desire for more aggressiveacceleration than would be available within the CC mode and thereforethe control of the present embodiment effectuates a controlled releaseto advantageously respond to such demand by providing the operator withthe torque multiplication benefits of additional fluidic coupling viathe hydrodynamics of the torque converter.

Upon initiation of the RELEASE mode at time t1, a RELEASE mode baselinefluid pressure (Prel) equivalent to the terminal value of CC mode netfluid pressure PTCC is established. Throughout the RELEASE mode thebaseline fluid pressure Prel remains fixed and an accumulation of fluidpressure adjustments Pramp is summed therewith to establish the netfluid pressure PTCC. All fluid pressure adjustments are equivalent inboth magnitude and algebraic sign, thereby effectuating a substantiallylinear reduction in the net fluid pressure PTCC. The rate of reductionis preferably at a rate greater than the CC mode rate since a quickerresponse is generally sought in the RELEASE mode. Additionally, a firstrate of PTCC reduction is utilized if the release is invoked by releaseline criteria and a second, more aggressive rate of PTCC reduction isutilized if the release is invoked by throttle rate of change. It isnoted that the net fluid pressure PTCC is not influenced in the RELEASEmode by any portion thereof being correlated to a measure of enginetorque. The RELEASE mode will normally continue with reductions to thenet pressure PTCC at the appropriate rate until the expiration of apredetermined amount of time or net fluid pressure PTCC equals zero,whichever occurs first.

In addition to normal transition to OFF mode, a fail-safe selection ofthe OFF mode is preferably included to detect appropriate releaseprogression. In the present embodiment, where slip ΔN does not exceed aRELEASE mode threshold ΔNrel within a predetermined amount of time orcontrol loops, then OFF mode is immediately selected.

FIGS. 18A-18C illustrate TCC slip, control chamber fluid pressure andvehicle speed, respectively, along common time axes through transitioninto and out of the LOCK mode of operation. As previously mentioned withrespect to FIG. 23, the LOCK mode is only accessible from the CC mode.The transmission is assumed to be in the highest available gear which isconventionally appropriate for TCC lock-up. Generally, vehicle speed inexcess of a first LOCK mode speed threshold (NvlockH) will causeapplication of a TCC in a conventional control system. In the presentcontrol, however, NvlockH is exceeded at time t1 without TCC lock-upimmediately ensuing. The LOCK mode is not invoked until slip ΔN, for apredetermined amount of time (t2-t3), remains less than or equal to theLOCK mode initiation slip threshold established as a summation of thereference slip ΔNref and a CC mode slip offset ΔNcc.

At the termination of the CC mode, accumulation of further fluidpressure adjustments ceases and the terminal value thereof is frozen.The net fluid pressure PTCC at the termination of the CC mode is summedwith a predetermined LOCK mode fluid pressure step (Plock) to establishthe initial net fluid pressure in the LOCK mode. Throughout the LOCKmode the net fluid pressure is comprised of the summation of a baselinefluid pressure having a predetermined correlation to engine torque Ptq,the frozen accumulation of fluid pressure adjustments Pramp and the LOCKmode pressure offset Plock.

The LOCK mode terminates normally and transitions to the CC mode whenvehicle speed crosses a second LOCK mode speed threshold (NvlockL) suchas is illustrated at time t4. An initial net fluid pressure in the CCmode is then established as the terminal net fluid pressure in the LOCKmode reduced by the LOCK mode fluid pressure step. CC mode net fluidpressure adjustments then continue as previously described.

Generally, the amount of fluid pressure added by the LOCK mode fluidpressure step is calibrated to be marginally sufficient to overcome therotational forces of the input member and maintain the TCC in a lock-upstate. All else being equal, a greater rotational force than thatpresent at the time of lock-up will be required to cause TCC slip due tothe larger coefficient of friction associated with static bodies. Thisprovides a inherent amount of hysteresis which prevents undesirable TCCslip in the case of marginal torque perturbations. However, it alsoprovides the TCC with the inherent ability to bypass more substantialpositive torque perturbations and adaptively adjust net fluid pressuresin the event of a LOCK mode slip.

An examination of FIGS. 19A and 19B demonstrate such benefits andfeatures of the present invention. If the LOCK mode is operational andmeasured slip ΔN, for a predetermined amount of time (t1-t2), remainsgreater than the LOCK mode termination slip threshold established as thesummation of the reference slip ΔNref and a LOCK mode slip offsetΔNlock, then CC mode is immediately invoked. However, in contrast to anormal transition to CC mode, where TCC slip invokes the transition outof LOCK mode (a slip transition), the initial net fluid pressure in theCC mode is then established as the terminal net fluid pressure in theLOCK mode without being reduced by the LOCK mode fluid pressure step. CCmode net fluid pressure adjustments then continue as previouslydescribed with the initial value of accumulation of fluid pressureadjustments Pramp equal to the frozen accumulation of fluid pressureadjustments Pramp and the LOCK mode fluid pressure step Plock. Thisadditional LOCK mode fluid pressure step being integrated into the netfluid pressure PTCC adaptively provides for torque capacity beneficialfor minimizing further slip deviations.

In order that reapplication of the TCC does not occur too soon after aslip transition, the LOCK mode is preferably forestailed for apredetermined period of time [t2-t3] after which LOCK mode may beinvoked if all other conditions are met. In the FIG. 19A, although slipΔN remains below the threshold (ΔNref+ΔNcc) for an otherwise adequatetime, the control of the present invention ignores such event until timet3 due to the previous slip transition and corresponding delay (t2-t3).In this FIG. 19A, the slip ΔN remains below the LOCK mode initiationslip threshold for the requisite time (t3-t4) and LOCK mode is onceagain invoked. The initial net pressure in LOCK mode is againestablished by adding to the terminal net fluid pressure in CC mode apredetermined LOCK mode fluid pressure step Plock.

Another mode of operation accessible from the CC mode and LOCK mode isCOAST mode. FIGS. 20A-20C illustrate slip, control chamber fluidpressure and throttle position, respectively, versus common time axesfor transition into and out of COAST mode and CC mode. COAST mode isinvoked in accordance with identical criteria from either CC mode orLOCK mode. Transition from the CC mode to the COAST mode is illustratedin FIGS. 20A-20C where throttle position crosses a first coast throttlethreshold % THcstL. Release of throttle position beyond this calibrationthreshold is likely to reduce positive engine torque significantly andresult in the vehicle drivetrain driving the engine. At the initiationof the COAST mode at t1, the terminal value of the accumulation of fluidpressure adjustments Pramp is saved as Psv and thereafter reset to zero.Also, the initial net fluid pressure PTCC is established as a COAST modebaseline fluid pressure Pcst which is a function of torque converteroutput member speed. Thereafter throughout COAST mode, the net fluidpressure PTCC is established as the summation of Pcst and theaccumulation of fluid pressure adjustments Pramp. These adjustments aremade in a similar fashion to those made in the CC mode; however, it isthe absolute value of slip ΔN that is compared to the reference sliplNref to establish the direction of adjustment. The object of slipcontrol in the COAST mode is to maintain torque capacity of the TCCsufficient to drive the engine with available wheel torque whileproviding damping for driveline disturbances such as those experiencedupon resumption of throttle position and associated torque reversal.

COAST mode normally transitions to CC mode upon throttle positionexceeding a second coast threshold (% THcstH). This second threshold isgreater than the first threshold and together therewith provides for adegree of hysteresis. FIG. 20C shows gradual resumption of throttleposition above the first coast throttle threshold % THcstL at a timeprior to t3 without causing transition to the CC mode. Anothertransition to CC mode occurs if slip ΔN exceeds a COAST mode slipthreshold ΔNcst for a predetermined time. This transition is illustratedin FIGS. 20A-20C where slip ΔN exceeds the threshold ΔN for therequisite time (t2-t3).

Upon transition to CC mode, the net fluid pressure is once againdetermined as the summation of a portion corresponding to positiveengine torque Ptq and a portion comprising the accumulation of fluidpressure adjustments Pramp. The initial net fluid pressure isestablished by summing Ptq with the value of Pramp saved as Psv at theinception of the COAST mode. Further adjustments to Pramp are then maderelative its initial CC mode value.

FIGS. 22A-22D illustrate a throttle change induced addition to net fluidpressure advantageously providing a slip margin to prevent unintendedfull application of the TCC or to provide additional torque couplingthrough the hydrodynamics of the torque converter. In essence, rapidapplication or release of the throttle results in the addition orsubtraction of a correlating signed pressure step Pdth from the netfluid pressure PTCC.

FIG. 22D shows a trace representing PTCC through a CC mode. At time t1 asufficient negative rate of throttle position change occurs and thesystem responds by adding a negative pressure step Pdth from PTCC.Similarly at times t2 and t3, sufficient positive rate of throttleposition changes occur and the system responds by subtracting from PTCCa positive pressure step Pdth. Preferably, the magnitude of Pdth isdetermined substantially in proportion to the magnitude of the throttleposition change.

The control function of the system as described above are carried out bythe transmission control unit 180 when it executes program instructionsrepresented by the flow diagrams shown in FIGS. 2-15. The instructionset is executed at regular intervals and may be part of a larger set ofinstructions for performing other vehicle control functions. Referringfirst to FIG. 2, step 201 is executed such as at the initiation ofvehicle operation to initialize various counters, flags, timers andvariables, and read in various calibration constants from ROM into RAMin preparation for performing the functions of the various vehiclecontrols, including those of the present invention. The table below setsforth various calibration constants as used in the present control:

    ______________________________________                                        CALIBRATION CONSTANTS                                                         CALIBRATION                                                                   CONSTANT    DESCRIPTION                                                       ______________________________________                                        ΔNrel RELEASE mode slip threshold                                       ΔNoff OFF mode slip threshold                                           ΔNcc  CC mode slip offset                                               ΔNcst COAST mode slip threshold                                         ΔNapp APPLY mode slip offset                                            % THcstL    coast throttle threshold low                                      % THcstH    coast throttle threshold high                                     % THmin     minimum throttle threshold                                        ETmin       minimum engine temperature                                        TRTmin      minimum transmission temperature                                  Δ % TH(-)                                                                           negative throttle change threshold                                Δ % TH(+)                                                                           positive throttle change threshold                                ΔN(-) negative slip threshold                                           NvcstL      COAST mode vehicle speed threshold                                NvlockH     LOCK mode vehicle speed threshold high                            NvlockL     LOCK mode vehicle speed threshold low                             RELRATEdth  RELEASE mode delta throttle fluid                                             pressure adjustment rate                                          RELRATEdef  RELEASE mode default fluid pressure                                           adjustment rate                                                   CCRATEdef   CC mode default fluid pressure                                                adjustment rate                                                   T           fluid pressure adjustment period                                  Plock       LOCK mode fluid pressure step                                     ______________________________________                                    

From step 201, steps 203-209, which represent various program stepsgermane to the present inventive control, are repetitively executed.Step 203 represents steps to read, filter, convert analog to digital andotherwise condition various inputs from transducers and sensors asutilized in the present control functions. The table below sets forthvarious inputs as used in the present control.

    ______________________________________                                        INPUT TABLE                                                                   INPUT      DESCRIPTION                                                        ______________________________________                                        RANGE      operator selected gear range (PRND32L)                             BR         brake signal                                                       Ne         engine speed                                                       Nt         turbine speed                                                      No         transmission output speed                                          % TH       throttle position                                                  ______________________________________                                    

Step 205, next encountered, represents program steps for repetitivelyupdating variables from the various inputs, calibration constants,calculations, and calibration table look-ups performed thereby. Includedin such updates are the following variables and correspondingdescriptions.

    ______________________________________                                        VARIABLE TABLE                                                                VARIABLE    DESCRIPTION                                                       ______________________________________                                        ΔN    measured slip (Ne-Nt)                                             Δ % TH                                                                              throttle position change                                          Nvrel       TCC release line                                                  Nvapp       TCC apply line                                                    APPRATE     rate of fluid pressure adjustments in                                         APPLY mode                                                        CCRATE      rate of fluid pressure adjustments in                                         CC mode                                                           CSTRATE     rate of fluid pressure adjustments in                                         COAST mode                                                        ΔNref reference slip                                                    Pdth        delta throttle pressure                                           Ptq         APPLY, CC & LOCK mode baseline fluid                                          pressure                                                          Pcst        COAST mode baseline fluid pressure                                Pramp       accumulated fluid pressure adjustments                            ______________________________________                                    

Step 207 represents operating mode selection and control calculationsconsistent therewith in accordance with the priority approach asillustrated in FIG. 3. Step 209 represents controller interface with thetransmission components controlled thereby including solenoid valve 72via line 182 as illustrated in FIG. 1.

FIG. 3 shows the priority selection logic for the various modes in thepresent control in accordance with appropriate bit settings of one ormore mode register(s) dedicated to that function and updatedperiodically as described herein. The first affirmative response to anystep 301-311 routes control to the corresponding mode steps, the step301-311 being illustrated with the preferred relative levels ofprioritization from top to bottom sequentially. In step 301, adetermination is made with respect to the highest priority mode, the OFFmode. A positive response to the query results in the execution of theOFF mode program steps represented by step 500 and the flow chart inFIG. 5, and thereafter bypassing further mode selection queries in thecurrent control loop via line 313. A negative response at step 301passes control to program steps represented by step 303 whereat adetermination is made with respect to the next highest priority mode,the RELEASE mode. If the RELEASE mode is indicated by appropriate bitsettings in the mode register(s), RELEASE mode program steps areexecuted as represented by step 700 and the flow chart of FIG. 7. Theremaining mode select queries are thereafter bypassed via line 313. Anegative response at step 303 of course results in a query at step 305with respect to the next highest priority mode, APPLY mode. Similar tothe immediately preceding description of priority routing to appropriatecontrol mode program steps, the APPLY, CC, LOCK and COAST mode programsteps are appropriately selected and executed at step pairs 305 & 900,307 & 1100, 309 & 1300, and 311 & 1500, respectively.

Turning to FIG. 5, a flow chart representing program steps executable inthe OFF mode are illustrated. It is the first mode discussed herein inas much as it is the default mode first executed. Here, step 501 setsPTCC to zero, PTCC representing net fluid pressure control signal andrelative net fluid pressure corresponding thereto. In the presentembodiment, a zero control signal setting represents a zero duty cyclePWM signal and corresponds to the minimum net fluid pressure and a 100%duty cycle corresponds to the maximum net fluid pressure. Thecorrelation between the PWM duty cycle and net fluid pressure istherefore positive. Negative correlations are equally applicable wherebya 100% duty cycle PWM control signal would correspond to the minimum netfluid pressure and a zero percent duty cycle to the maximum net fluidpressure. For simplicity in description and correspondence between theflow charts and graphic illustrations contained herein, furtherdescription will be with respect to positively correlated PWM controlsignals and net fluid pressures.

Step 503 initializes the accumulation of fluid pressure adjustment Prampto zero, thus ensuring that when the OFF mode is exited to the APPLYmode an initial value for fluid pressure adjustments will always bezero. Steps 505 and 507 represent program steps for evaluating variouscriteria to determine the propriety of transitioning out of the OFF modeinto the APPLY mode. Step 505 represents program steps detailed in FIG.4 executed to determine whether various OFF mode criteria continue toindicate the desirability of remaining in the OFF mode. Where any of theOFF mode criteria indicates the continuance of the OFF mode,determinations of the desirability of the lower priority APPLY mode issuperfluous and therefore will be bypassed via line 506. Where none ofthe OFF mode criteria indicates the continuance of the OFF mode, step507 is encountered to determine whether the APPLY mode is to beinitiated. A flow chart representing program steps executed to determineif various APPLY mode criteria indicate the desirability of initiatingthe APPLY mode is illustrated in FIG. 8. Where such a transition to theAPPLY mode is not warranted based on the criteria, the mode continues tobe specified as OFF mode. The program steps represented by steps 505 and507 first to specify affirmative selection of a respective mode willdetermine the mode to be invoked in the next control loop.

The OFF mode check is performed in accordance with the steps shown inFIG. 4. Step 401 determines whether the current mode from which this OFFmode check is occurring is the RELEASE mode. If RELEASE mode iscurrently active then steps 403-413, which represent criteria checksuniquely related to the RELEASE mode to OFF mode transition, determineif (a) slip ΔN has exceeded the RELEASE mode slip threshold ΔNrel withina predetermined maximum release delay time, (b) RELEASE mode has beenoperative for the maximum allowable time, or (c) net fluid pressure isminimum. Any of these criteria being met will bypass the remaining OFFmode criteria checks via line 437 and set appropriate bits in the moderegister(s) at step 435. Thereafter, control returns to the appropriateportion of the program via the branch labeled "YES".

Timer flag "RELTMRFL" at step 403 is checked to determine if slip ΔN hasexceeded the RELEASE mode threshold ΔNrel within a predetermined maximum"release delay" time. RELTMRFL equal to zero indicates that the slip ΔNin the previous loop did not exceeded the threshold and another check ofslip in the present loop is required to be performed at step 405. SlipΔN is checked once again against the threshold slip ΔNREL and if it doesnot exceed the threshold step 411 checks to see if the timer "RELTMR"has expired. RELTMR, for purposes of this check, was initialized at amaximum release delay time at the onset of the RELEASE mode. If RELTMRexpires before slip ΔN crosses the thresholdNrel, the OFF mode isimmediately selected.

Where the result of the query in step 405 is affirmative, indicatingslip ΔN has exceeded the threshold ΔNrel within the time allotted, it isassumed that RELEASE mode progression is normal and RELTMR isinitialized to a maximum allowable time for continuing the RELEASE modeat step 407. Step 409 then sets the RELTMRFL to "1" so that futurepasses through the OFF mode check during the RELEASE mode will skipsteps 405-409 and proceed directly step 411 whereat RELTMR is checkedfor expiration, such expiration being indicative of the maximumallowable time for RELEASE mode operation having been exceeded.Therefore, an expired RELTMR from this point forward results in OFF modeselection. An unexpired RELTMR, whether quantifying maximum releasedelay or maximum time in RELEASE mode, routes further processing to step413 whereat net fluid pressure is checked. If net fluid pressure isdetermined to be at a minimum, RELEASE mode is completed and OFF mode isselected via line 437 and step 435. Where none of the criteria of steps403-413 result in selection of the OFF mode, further checks of OFF modecriteria not uniquely related to the transition from RELEASE mode to OFFmode are performed (steps 419-433).

If step 401 determines that the RELEASE mode is not the current modefrom which the present OFF mode checks are being performed, step 415 isexecuted to make a similar determination with respect to the APPLY mode.An affirmative response to step 415 routes control to a step 417 whereinthe throttle position % TH is checked against a coast throttle threshold% Thcstl. A throttle setting less than the threshold causes immediateselection of the OFF mode form the APPLY mode since continuedapplication of the TCC where a coast condition is indicated is notdesirable. A negative response at step 415 or a throttle position abovethe coast throttle threshold while in APPLY mode results in continuedchecks of OFF mode criteria not uniquely related to the transition fromAPPLY mode to OFF mode via steps 419-433.

Steps 419-433 represent generic OFF mode criteria, any one of which ifaffirmative causes immediate selection of the OFF mode. Step 419 is aminimum throttle setting check whereat current throttle position % TH ischecked against a threshold represented by % Thmin. The minimum throttlesetting check is caused to occur with a degree of hysteresis based on apair of throttle thresholds comprising % Thmin. Step 421 is a check ofoperator selected gear range "RANGE". In the present embodiment asimplemented in a four speed automatic transmission, selection of a rangeof gears limited to first and second will ensure operation in the OFFmode. Engine temperature and transmission temperature minimums arechecked at steps 423 and 425 respectively to ensure closed loop enginecontrol and appropriate transmission operating fluid temperatures foraccurately and repeatably performing the control functions of thepresent invention. Service brake application is checked at step 427,which application will cause immediate selection of the OFF mode. Thethrottle position change Δ% TH is checked against a threshold Δ% TH(-)to determine if the operator has released the throttle abruptly enoughto warrant OFF mode selection. Step 431 selects the OFF mode when a downshift is in progress and step 433 selects the OFF mode if slip ΔN isnegative and less than a negative slip threshold ΔN(-).

Referring to FIG. 6, various RELEASE mode criteria checks are performedin accordance with steps 601-609. A mode check is first performed atstep 601 to determine whether COAST mode is active. Where COAST mode isactive, step 603 is performed to determine if the vehicle speed is belowa COAST mode vehicle speed threshold NvcstL thus indicating thedesirability of exiting the COAST mode. Where COAST mode is not activeor, where active, the vehicle speed threshold NvcstL is not crossed,step 605 is executed. Step 605 determines from the release line criteriawhether the TCC should be released. If so, step 609 is executed toinvoke the RELEASE mode. The last criteria in the RELEASE mode checkdetermines if the rate of throttle position change Δ% TH exceeds apositive threshold Δ% TH(+) which would be consistent with operatordemand for relatively aggressive acceleration and therefore a controlledrelease into the OFF mode where increase fluidic coupling willadvantageously provide torque multiplication.

Where the RELEASE mode is the mode selected, the steps illustrated inFIG. 7 are executed to perform the control functions of the RELEASEmode. A first set of steps 701-719 are mode initialization steps; asecond set of steps 721-723 are fluid pressure control and calculationsteps; and, a third set of steps 725-731 are mode transition controlsteps. Step 701 is first encountered and determines whether RELEASE modehas just been initiated. A first time pass through the program stepswill route control to steps 703-717 to initialize certain variables andtimers used in the RELEASE mode control. RELTMR is initialized at amaximum release delay time at step 703 and RELTMRFL is initialized atzero. The timer and corresponding flag are used in the OFF mode check aspreviously described. The accumulation of fluid pressure adjustmentPramp is initialized to zero at step 707 and RELEASE mode baseline fluidpressure Prel is set, at step 709, to the terminal PTCC value from themode being exited. Prel remains fixed thereafter throughout the RELEASEmode. Another timer, Δ% THTMR, is initialized at step 711 for use indelaying reapplication of the TCC too soon after initiating the RELEASEmode. Step 713 determines if the release is the result of a rate ofthrottle position change Δ% TH exceeding the positive threshold Δ%TH(+). If the release is due to a high rate of change in throttleposition, step 715 sets the rate "RELRATE" at which accumulation offluid pressure adjustments will occur in the RELEASE mode to a RELEASEmode delta throttle rate value "RELRATEdth". If, however, the release isnot due to a high rate of change in throttle position, step 717 setsRELTATE to a RELEASE mode default rate value "RELRATEdef". Due to thenature of a throttle change RELEASE mode request, RELRATEdth iscalibrated greater than RELRATEdef in order the torque multiplicationattributes of the fluidic coupling are swiftly invoked upon theoperator's demand. Next executed is step 719 wherein the unit fluidpressure adjustment (Pinc) is determined as a function of RELRATE andthe period "T" of the control loop wherein adjustments are accumulated.Steps 721-731 are next executed as discussed below.

Passes subsequent the first pass through the RELEASE mode steps willbegin at step 721 wherein the accumulation (Pramp) of fluid pressureadjustments (Pinc) occurs. In the RELEASE mode, the accumulation offluid pressure adjustments is always such that Pramp represents adecreasing fluid pressure component of the net fluid pressure. Step 723sets the net fluid pressure as the summation of the RELEASE modebaseline fluid pressure Prel and the accumulation of fluid pressureadjustments Pramp. The static nature of the baseline fluid pressure Preland the decreasing nature of the accumulation of adjustments results ina substantially linear reduction in net fluid pressure through theRELEASE mode of operation in accordance with the rate selected at steps713-717.

Steps 725-731 are next executed and represent the portion of RELEASEmode that controls transition thereof and initiation of an appropriatenext mode. The OFF mode check as illustrated in FIG. 4 is executed atstep 725 with an affirmative return therefrom resulting in bypassing theremaining steps 727-731 via line 733. A negative return from OFF modecheck results in execution of step 727. If the RELEASE mode was enteredbecause of a throttle change, step 729 checks if Δ% THTMR has expired,after which the APPLY mode check may be executed at step 731. If athrottle change did not invoke the present RELEASE mode, step 729 isbypassed and step 731 is immediately executed. RELEASE mode continueswhere no affirmative return from either mode checks 725,731 results fromtheir respective executions.

Turning to FIG. 8, steps comprising the APPLY mode check areillustrated. Step 801 represent a determination whether the vehiclespeed Nv has crossed the apply line as represented by Nvapp. A negativeresponse does not result in the selection of the APPLY mode. Anaffirmative response results in execution of step 803 whereat measureslip ΔN is compared to the OFF mode slip threshold ΔNoff which, ifexceeded, results in the selection of the APPLY mode at step 805.Measured slip below the threshold will not result in selection of theAPPLY mode.

FIG. 9 illustrates steps executed during the APPLY mode. Again, a firstset of steps 901-903 are mode initialization steps; a second set ofsteps 905-909 are fluid pressure control and calculation steps; and, athird set of steps 911-915 are mode transition control steps. Beginningat step 901, if the APPLY mode has just been initiated, step 903 isexecuted to initialize Pramp to zero; otherwise, control passes to step905. Step 905 is then executed wherein the unit fluid pressureadjustment (Pinc) is determined as a function of APPRATE and the period"T" of the control loop wherein adjustments are accumulated. APPRATE isnot static throughout the APPLY mode; rather, APPRATE is preferably afunction of vehicle speed and looked up from a calibration table duringan update of variables as described with respect to step 205 in FIG. 2.Preferably, the value of APPRATE will increase with increasing vehiclespeeds as higher vehicle speeds tend to require larger applicationforces to maintain slip and therefore more aggressive rates to maintaina slip reduction in a desired time. Accumulation (Pramp) of fluidpressure adjustments (Pinc) occurs at step 907, the accumulation in theAPPLY mode being such that Pramp represents an increasing fluid pressurecomponent of the net fluid pressure. Step 909 next establishes the netfluid pressure PTCC as the summation of an APPLY mode baseline fluidpressure Ptq and the accumulation of fluid pressure adjustments Pramp.Ptq is the portion of the net fluid pressure represented as a functionof engine torque and as such will be dynamic in its magnitude. Asubstantially constant engine torque will result in a PTCC which variesonly with Pramp, thereby resulting in a substantially linear increasethroughout such constant torque intervals in the APPLY mode ofoperation. However, in the event torque increase or decreases during theAPPLY mode, PTCC will vary in accordance therewith. Such control of PTCCmay result in increased or decreased rates of PTCC increases, or evendecreases in PTCC during the APPLY mode. PTCC is thereby adjusted notonly in response to the sense of slip but additionally in response tothe actual engine torque.

Steps 911-915 are next executed to perform OFF mode, RELEASE mode and CCmode checks as illustrated in detail in FIGS. 3, 6 and 10 respectively.Again, an affirmative return from any of the checks will result inselection of the corresponding mode and transition out of the APPLYmode. The first of the checks 911-915 resulting in an affirmative returnwill bypass the remaining checks via line 917.

Referring to the CC mode check as illustrated in FIG. 10, steps1001-1027 are executed when the CC mode check is performed from variousmodes of operation. The entry into CC mode may occur from the APPLYmode, LOCK mode or COAST mode and steps 1001 and 1013 determine routingto appropriate steps within the CC mode check in accordance with the oneof those modes executing the CC mode check. Step 1001 routes control tosteps 1003-1011 if the CC mode check is caused to occur from the LOCKmode. Step 1013 routes control to steps 1015 and 1017 if the CC modecheck is caused to occur from the APPLY mode. And, steps 1019-1025 arecaused to occur by default where the CC mode check is caused to occurfrom the COAST mode.

Assuming that the current mode is the APPLY mode, step 1015 determineswhether measured slip ΔN exceeds the APPLY mode slip thresholdestablished as a summation of the reference slip ΔNref and the APPLYmode slip offset ΔNapp. An affirmative response to step 1015 indicatesthat the slip is sufficiently close to the reference slip to begin theCC mode and selects the CC mode at step 1027. A negative response atstep 1015 results in the execution of step 1017 wherein the net fluidpressure PTCC is checked against a maximum allowable fluid pressurethreshold (PTCCmax). Further adjustments in the APPLY mode to PTCCcannot be made if PTCCmax has been attained or exceeded and, therefor,CC mode will immediately be invoked based upon a maximum PTCC viaexecution of step 1027. An affirmative response at either steps 1015 or1017 results in CC mode selection and an affirmative return to the modetransition control steps of the APPLY mode. Of course, negativeresponses to both steps 1015 and 1017 results in a negative return tothe mode transition control steps of the APPLY mode without selection ofthe CC mode.

Assuming that the current mode is the LOCK mode, step 1003 determines ifvehicle speed Nv exceeds a LOCK mode speed threshold NvlockL. If thislower limit on vehicle speed is no longer exceeded, then CC mode isselected at step 1027 and the CC mode check returns with an affirmativeresponse to the mode transition control steps of the LOCK mode. Wherethe lower limit on vehicle speed continues to be exceeded, Pramp isinitialized at the terminal value of PTCC in the LOCK mode. Step 1005compares measured slip ZN to the LOCK mode termination slip thresholdestablished as the summation of the reference slip ΔNref and the LOCKmode slip offset ΔNlock. If the threshold is not exceeded, then step1009 initializes a timer "LKOFFTMR" which, if allowed to expire,indicates that slip has exceeded the LOCK mode termination slipthreshold for a period of time sufficient to warrant return to CC modein order to regain control of the TCC slip. LKOFFTMR is continuallyinitialized in the LOCK mode by step 1009 so long as slip does notexceed the threshold at step 1005. Where slip does exceed the threshold,step 1007 determines if LKOFFTMR has expired. If it has not, thencontrol returns via a negative return path to the mode transitioncontrol steps of the LOCK mode. Continuous operation in the LOCK modewherein the slip threshold is exceeded results in expiration of LKOFFTMRand setting of a lock slip flag "LKSLFL" at step 1011, immediateselection of the CC mode at step 1027 and return via an affirmativereturn path to the mode transition control steps of the LOCK mode.LKSLFL will be utilized during the CC mode to establish certain initialcontrol values and to forestall immediate selection of the LOCK mode.

Where the CC mode check is performed from the COAST, step 1019 firstdetermines if the throttle position % TH exceeds the coast threshold (%Thcsth). Such a throttle position is indicative of the operator resumingpositive engine torque delivery to the vehicle drivetrain and as suchcauses selection of CC mode at step 1027. Where the throttle position isstill below the threshold, step 1021 is executed to determine if slipexceeds the COAST mode slip threshold ΔNcst. A negative responseinitializes a time CSTTMR at step 1025 which, if allowed to expire,indicates that slip has exceeded the COAST mode slip threshold for aperiod of time sufficient to warrant return to CC mode in order toregain control of the TCC slip. CSTTMR is continually initialized in theCOAST mode by step 1025 so long as slip does not exceed the threshold atstep 1021. Where slip does exceed the threshold, step 1023 determines ifCSTTMR has expired. If it has not, then control returns via a negativereturn path to the mode transition control steps of the COAST mode.Continuous operation in the COAST mode wherein the slip threshold isexceeded results in expiration of CSTTMR, immediate selection of the CCmode at step 1027 and return via an affirmative return path to the modetransition control steps of the COAST mode.

CC mode is next described with reference to the steps illustrated inFIG. 11. A first set of steps 1103-1121 are mode initialization steps; asecond set of steps 1123-1139 are fluid pressure control and calculationsteps; and, a third set of steps 1141-1147 are mode transition controlsteps.

Beginning at step 1101, if the CC mode has just been initiated, steps1103 and 1105 are executed to initialize a timer "ONTMR" and slip flag"CCSLFL, respectively, for utilization in the identification and controlof excessive slip during an initial period of CC mode operation. Step1107 initializes a timer "LKONTMR" utilized in the LOCK mode check stepsof FIG. 12 to control transition into the LOCK mode from the CC mode.

If the CC mode was invoked from the LOCK mode, step 1109 routes controlto step 1111 whereat the lock slip flag LKSLFL is checked to determineif the LOCK mode was exited due to excessive slip. An affirmativeresponse at step 1111 indicates that excessive slip caused thetransition to the CC mode and delay timer "LKSLTMR" is initialized toforestall LOCK mode selection in accordance with the delay at step 1115.Since an excessive slip was determined to cause the transition from LOCKmode to CC mode, Pramp is initialized at the terminal value of net fluidpressure PTCC in the LOCK mode at step 1119 to advantageously capitalizeon the additional capacity afforded by the LOCK mode fluid pressureoffset Plock. Control continues to the fluid pressure control andcalculation steps at step 1130.

If the CC mode was invoked from the COAST mode, step 1113 routes controlto step 1117 whereat Pramp is initialized at the value Psv which isequivalent to the value of Pramp saved at the inception of the COASTmode. Control then continues to the fluid pressure control andcalculation steps at step 1130.

If the CC mode was invoked from the APPLY mode, step 1121 is executed toinitialize Pramp at the terminal value of Pramp in the APPLY mode. Onceagain, control continues to the fluid pressure control and calculationsteps at step 1130.

Passes subsequent the first pass through the CC mode steps will begin atstep 1123 wherein ONTMR is checked for expiration. Where ONTMR has notyet expired, step 1127 is executed to determine if slip ΔN exceeds theAPPLY mode slip threshold established as a summation of the referenceslip ΔNref and the APPLY mode slip offset ΔNapp. This condition may beexperienced where an increasing slip is present upon initiation of theCC mode. Where the threshold is exceeded, slip flag CCSLFL is set atstep 1131 for future use in selecting an appropriate fluid pressureadjustment rate. The rate "CCRATE" at which accumulation of fluidpressure adjustments will occur in the CC mode is then set to APPRATE.APPRATE represents more aggressive fluid pressure adjustments than a CCmode default rate "CCPATEdef" otherwise utilized. Additionally, sinceAPPRATE is variable in accordance with vehicle speed, too aggressivefluid pressure adjustments are not likely. APPRATE will continue to beused in the CC mode until slip is controlled below the threshold(ΔNref+ΔNapp).

Referring back to step 1123, where ONTMR has expired, step 1125 isexecuted. If the slip flag CCSLFL was set previously, then slip willcontinue to be checked at step 1127. Once slip is under control, step1127 will route control to step 1129 whereat CCSLFL is reset to preventfurther usage of the more aggressive fluid pressure adjustments.Therefor, where ONTMR has expired and CCSLFL is reset, step 1130 setsCCRATE to the default CCRATEdef.

After steps 1123-1133 determine the appropriate CCRATE, step 1135 isexecuted wherein the unit fluid pressure adjustment (Pinc) is determinedas a function of CCRATE and the period "T" of the control loop whereinadjustments are accumulated. At step 1137, Pramp is established as theprevious Pramp and an addition or subtraction of Pinc where the measuredslip ΔN is above or below the reference slip ΔNref respectively.Additionally, an adjustment to PTCC may be made in accordance with achange in throttle position to ensure that rapid throttle positionchanges do not cause undesirable full application of the TCC in the caseof stepping out of the throttle and to provide for an additional slipmargin in the case of stepping into the throttle. By providingadjustment to PTCC in response to negative throttle changes, the controlmakes downward adjustments to PTCC by a calibration substantiallyproportional to the rapidity of the throttle change thereby providing anadditional margin of slip by virtue of a stepwise pressure reduction toprevent unintended full application of the TCC. Similarly, by providingadjustment to PTCC in response to positive throttle changes, the controlalso makes downward adjustments to PTCC by a calibration substantiallyproportional to the rapidity of the throttle change thereby providing anadditional margin of slip by virtue of a stepwise pressure reduction toprovide additional torque coupling and attendant torque multiplicationthrough the hydrodynamics of the torque converter. This adjustment isshown as a delta throttle pressure "Pdth" in step 1137. Ultimately, step1139 establishes PTCC as the summation a CC mode baseline fluid pressurePtq and the accumulation of fluid pressure adjustments Pramp. Asdiscussed with reference to the APPLY mode, Ptq is the portion of thenet fluid pressure represented as a function of engine torque and assuch will be dynamic in its magnitude. A substantially constant enginetorque will result in a PTCC which varies only with Pramp, therebyresulting in a substantially linear increase throughout such constanttorque intervals in the CC mode of operation. However, in the eventtorque increase or decreases during the CC mode, PTCC will vary inaccordance therewith. Such control of PTCC may result in increased ordecreased rates of PTCC increases, or even decreases in PTCC during theCC mode. PTCC is thereby adjusted not only in response to the sense ofslip but additionally in response to the actual engine torque.

Steps 1141-1147 are next executed to perform OFF mode, RELEASE mode,LOCK mode and COAST mode checks as illustrated in detail in FIGS. 3, 6,12 and 14 respectively. An affirmative return from any of the checkswill result in selection of the corresponding mode and transition out ofthe CC mode. The first of the checks 1143-1147 resulting in anaffirmative return will bypass the remaining checks via line 1149.

Referring to FIG. 12, steps comprising the LOCK mode checks areillustrated. These steps are executable only from the CC mode since theLOCK mode in the present embodiment is accessible only therefrom. Step1201 is first executed whereat LKSLFL is checked to determine if themost recent transition out of the LOCK mode into the CC mode was due toexcessive slip. An affirmative response at step 1201 indicates thatexcessive slip caused the transition to the CC mode and delay timerLKSLTMR is therefore checked for expiration at step 1203. An unexpiredLKSLTMR indicates that it is premature to initiate LOCK mode andtherefore will bypass further LOCK mode checks and exit the LOCK modecheck via the negative return path. Where, however, LKSLTMR has expired,LKSLFL is reset to allow future processing through the LOCK mode checkto proceed. Therefore, step 1207 is executed in this and future passesthrough the LOCK mode check steps.

Step 1207 represents the first criteria check for entering the LOCK modewhereat vehicle speed Nv is compared to the LOCK mode vehicle speedthreshold NvlockH. A negative response threat will initialize timerLKONTMR in response to the vehicle speed dropping below the threshold.If step 1207 is affirmatively answered, the vehicle speed is above thethreshold and the timer LKONTMR is allowed to continue towardexpiration. Step 1209 then checks the slip against the LOCK modeinitiation slip threshold established as the summation of the referenceslip ΔNref and the CC mode slip offset ΔNcc. If this too results in anaffirmative response, step 1213 is executed. However, is step 1209results in a negative response where slip is too high for LOCK modeactivation, step 1211 initializes LKONTMR. Step 1213, which is reachedupon satisfactory criteria checks at steps 1207 and 1209, check if theLKONTMR has expired. An expired timer threat indicates that the criteriacheck have passed for an adequate period of time and that LOCK mode maybe initiated. Therefore, step 1215 is allowed to set the mode to LOCKmode and return control via the affirmative return path to the modetransition control steps in the CC mode.

The LOCK mode steps are illustrated in FIG. 13 and comprise a set ofinitialization steps 1301-1307; a fluid pressure control and calculationstep 1309; and, a set of mode transition control steps 1311-1317.

Beginning at step 1301, if the LOCK mode has just been initiated, steps1303-1307 are executed to, respectively, save the terminal value ofPramp, in the CC mode from which the transition to LOCK mode occurred,initialize the timer LKOFFTMR, and reset the lock slip flag LKSLFL.

PTCC is then established at step 1309 as the summation of Pramp, theLOCK mode baseline fluid pressure Ptq and a LOCK mode fluid pressurestep Plock. Again, the baseline fluid pressure Ptq is the portion of thenet fluid pressure represented as a function of engine torque and assuch will be dynamic in its magnitude. A substantially constant enginetorque will result in a PTCC which remains constant throughout the LOCKmode since the other pressure contributions, Pramp and Plock, are staticthroughout the LOCK mode. However, in the event torque increase ordecreases during the LOCK mode, PTCC will vary in accordance therewith.Such control of PTCC may result in PTCC increases, or decreases in PTCCduring the LOCK mode. PTCC is thereby adjusted only in response to theactual engine torque.

Steps 1311-1317 are next executed to perform OFF mode, RELEASE mode,COAST mode and CC mode checks as illustrated in detail in FIGS. 3, 6, 14and 10 respectively. An affirmative return from any of the checks willresult in selection of the corresponding mode and transition out of theLOCK mode. The first of the checks 1311-1317 resulting in an affirmativereturn will bypass the remaining checks via line 1319.

The steps illustrated in FIG. 14 are executed as the COAST mode checkfrom the CC mode or the LOCK mode. Step 1401 determines the propriety ofentering the COAST mode by comparing the present throttle position % THto the COAST throttle threshold % THcstL. This minimum throttle setting,if crossed, will result in an affirmative response at block 1401 andselection of the COAST mode at step 1403 and thereafter return via theaffirmative return line to the transition control steps of the one ofthe CC and LOCK modes executing the COAST mode checks. A negativeresponse to step 1401 returns via the negative return line.

The COAST mode steps are illustrated with reference to FIG. 15. A set ofinitialization steps 1501-1509, a set of fluid pressure control andcalculation steps 1511 and 1513, and a set of mode transition controlsteps 1515-1519 comprise the COAST mode. Step 1501 is first executedand, at the initial pass through the steps, routes control to step1503-1509 to establish initial variable and timer values as utilizedthroughout the remaining passes through the COAST mode steps. Psv isinitialized at the terminal value of Pramp in the CC mode or LOCK modeat transition therefrom at step 1503. Pramp is then set to zero at step1505 so that accumulations will begin from that value. Timer CSTTMR,used in conjunction with CC mode transitions as detailed in FIG. 10, isinitialized at step 1507, and the CSTRATE at which accumulation of fluidpressure adjustments will occur in the COAST mode is then set to thedefault value for the accumulation of fluid pressure adjustments in theCC mode, i.e. CSTRATEdef. Thereafter, the unit fluid pressure adjustment(Pinc) is determined as a function of CSTRATE and the period "T" of thecontrol loop wherein adjustments are accumulated.

Control next passes to step 1511 wherein Pramp is recalculated with theaddition or subtraction of Pinc depending upon the magnitude of themeasured slip with respect to the reference slip so as to control slip,both negative and positive, during the COAST mode. From here, step 1513establishes the net fluid pressure PTCC as the summation of Pramp and aCOAST mode baseline fluid pressure "Pcst" which is a function of torqueconverter output member speed.

The transition control steps 1515-1519 are next executed and the firstof such steps encountered to be affirmatively answered cause bypass ofthe remaining steps.

While the present invention has been described with respect to certainpreferred embodiments, it is to be understood that various modificationsand alternatives thereto may be readily practiced within the scope andspirit of the invention which is defined by the appended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. In a motor vehicleincluding a fluidic torque converter adapted to transmit torque betweenan input member thereof coupled to an engine output shaft and an outputmember thereof coupled to a vehicle drivetrain, the torque beingpositive when the engine is driving the vehicle drivetrain and negativewhen the vehicle drivetrain is driving the engine, a variable torquecapacity clutch mechanism connected between the input and output membersof said torque converter wherein clutch slip is positive when the inputmember rotational speed exceeds the output member rotational speed andnegative when the output member rotational speed exceeds the inputmember rotational speed, and a clutch actuating mechanism forcontrolling the torque capacity of said clutch according to the netfluid pressure differential thereacross, a torque converter clutchcontrol comprising:control means for establishing a pressure controlsignal in response to a plurality of vehicle control inputs forcontrolling the clutch actuating mechanism in one of a plurality ofmodes of operation; an off mode of operation wherein said pressurecontrol signal causes substantially zero pressure differential acrosssaid clutch mechanism to cause substantially zero clutch torquecapacity; an apply mode of operation operable during periods of positivetorque for transitioning from the off mode of operation to a controlledcapacity mode of operation, wherein during said apply mode of operationthe pressure control signal varies to cause positive slip to decreasetoward a reference slip; a controlled capacity mode of operation whereinthe pressure control signal varies to increase and decrease the pressuredifferential across the clutch mechanism to maintain the clutchmechanism slip substantially at said reference slip, said pressurecontrol signal comprising a summation of a torque adaptive quantitywhich value varies with a predetermined correlation to a measure ofengine output torque and a slip adaptive quantity whose value varies ina first predetermined direction and a second predetermined directionwhen the clutch mechanism positive slip is respectively greater than andless than the reference slip; and a release mode of operation fortransitioning from the controlled capacity mode of operation to the offmode of operation wherein during said release mode of operation thepressure control signal varies to decrease the pressure differentialacross the clutch mechanism to cause clutch torque capacity to decrease.2. A torque converter clutch control as claimed in claim 1 furthercomprising a lock mode of operation accessible from said controlledcapacity mode of operation wherein during said lock mode of operationthe pressure control signal controls the pressure differential acrosssaid clutch mechanism to provide torque capacity at least as great asthe torque being transmitted between said input and output members, saidpressure control signal comprising a summation of said torque adaptivequantity, the controlled capacity mode of operation final value of saidslip adaptive quantity, and a predetermined lock quantity, and whereinthe release mode of operation is further adapted for transitioning fromthe lock mode of operation to the off mode of operation.
 3. A torqueconverter clutch control as claimed in claim 1 further comprising acoast mode of operation accessible from said controlled capacity mode ofoperation wherein during said coast mode of operation the pressurecontrol signal varies to increase and decrease the pressure differentialacross the clutch mechanism to maintain the absolute value of clutchmechanism slip substantially at said reference slip, said pressurecontrol signal comprising a summation of a drivetrain member speedadaptive quantity whose value varies with a predetermined correlation toa measure of a drivetrain member speed and a slip adaptive quantitywhose value varies in a first predetermined direction and a secondpredetermined direction when the absolute value of clutch mechanism slipis respectively greater than and less than the reference slip.
 4. Atorque converter clutch control as claimed in claim 1 wherein duringsaid apply mode of operation the pressure control signal comprises asummation of said torque adaptive quantity and an apply quantity thatvaries monotonically at a predetermined rate.
 5. A torque converterclutch control as claimed in claim 1 wherein during said controlledcapacity mode of operation the pressure control signal further comprisesin summation therewith a transient throttle quantity that varies with apredetermined correlation to the rate of change in throttle position. 6.In a motor vehicle including a fluidic torque converter adapted totransmit torque between an input member thereof coupled to an engineoutput shaft and an output member thereof coupled to a vehicledrivetrain, the torque being positive when the engine is driving thevehicle drivetrain and negative when the vehicle drivetrain is drivingthe engine, a variable torque capacity clutch mechanism connectedbetween the input and output members of said torque converter whereinclutch slip is positive when the input member rotational speed exceedsthe output member rotational speed and negative when the output memberrotational speed exceeds the input member rotational speed, and a clutchactuating mechanism for controlling the torque capacity of said clutchaccording to the net fluid pressure differential thereacross, a torqueconverter clutch control comprising:control means for establishing apressure control signal in response to a plurality of vehicle controlinputs for controlling the clutch actuating mechanism in one of aplurality of modes of operation; an off mode of operation wherein saidpressure control signal causes substantially zero pressure differentialacross said clutch mechanism to cause substantially zero clutch torquecapacity; an apply mode of operation operable during periods of positivetorque for transitioning from the off mode of operation to a controlledcapacity mode of operation, wherein during said apply mode of operationthe pressure control signal varies to increase the pressure differentialacross the clutch mechanism to cause clutch torque capacity to increaseand positive slip to decrease toward a reference slip; a controlledcapacity mode of operation wherein the pressure control signal varies toincrease and decrease the pressure differential across the clutchmechanism to maintain the clutch mechanism slip substantially at saidreference slip, said pressure control signal comprising a summation of atorque adaptive quantity whose value varies with a predeterminedcorrelation to a measure of engine output torque and a slip adaptivequantity whose value varies in a first predetermined direction and asecond predetermined direction when the clutch mechanism positive slipis respectively greater than and less than the reference slip; a lockmode of operation accessible from said controlled capacity mode ofoperation wherein during said lock mode of operation the pressurecontrol signal controls the pressure differential across said clutchmechanism to provide torque capacity at least as great as the torquebeing transmitted between said input and output members, said pressurecontrol signal comprising a summation of said torque adaptive quantity,the controlled capacity mode of operation final value of said slipadaptive quantity, and a predetermined lock quantity; a coast mode ofoperation accessible from said controlled capacity and lock modes ofoperation wherein during said coast mode of operation the pressurecontrol signal varies to increase and decrease the pressure differentialacross the clutch mechanism to maintain the absolute value of clutchmechanism slip substantially at said reference slip, said pressurecontrol signal comprising a summation of a drivetrain member speedadaptive quantity whose value varies with a predetermined correlation toa measure of a drivetrain member speed quantity and a slip adaptivequantity whose value varies in a first predetermined direction and asecond predetermined direction when the absolute value of clutchmechanism slip is respectively greater than and less than the referenceslip; and a release mode of operation for transitioning from thecontrolled capacity, lock and coast modes of operation to the off modeof operation wherein during said release mode of operation the pressurecontrol signal varies to decrease the pressure differential across theclutch mechanism to cause clutch torque capacity to decrease.